1. Field of the Invention
The field of invention relates to variable damping devices for protecting a structure from vibrational disturbances by varying the rigidity of the structure and/or its natural vibrational periods.
2. Description of the Prior Art
Prior art FIGS. 9 through 12 show examples of variable damping and variable stiffness structures provided with variable damping devices connected to frames of structures. As shown in prior art FIG. 9, frame 31 comprises columns 33, beams 34, and inverted V-type braces 35. The response in terms of amplitude, velocity, and acceleration of a structure at the time of an earthquake or other vibrational disturbance is detected by a response sensor 36 installed on the structure. The sensor 36 transmits a signal to a computer 14 which analyzes the signal and sends an appropriate command to variable damping device 1 to vary the rigidity of the structure as required.
This prior art variable damping device 1 is also shown in FIG. 10 to include a piston rod 4, a hydraulic cylinder 2, a flow regulating valve 12, and a hydraulic accumulator 19, having a predetermined capacity, in close proximity to the flow regulating valve 12. As shown in FIG. 11, a bearing shaft 22 and both ends of the piston rod 4 are connected to a bracket 34A installed on the beam 34 and to braces 35.
FIG. 12 is a schematic diagram of a hydraulic circuit for the prior art damping device 1, showing hydraulic chambers 6L and 6R installed to the left and right sides of a piston 3 with a piston rod 4 reciprocating within the hydraulic cylinder 2. The piston 3 is fixed or made movable to the right or left by stopping or flowing the hydraulic oil within these left and right hydraulic chambers 6 by a flow regulating valve 12. Flow regulating valve 12 selectively interconnects hydraulic chambers 6L and 6R. The operation of the flow regulating valve 12 is finely adjusted to provide various damping coefficients best suited to attenuate the seismic vibrations impacting on a structure, as more fully disclosed in U.S. Pat. Nos. 5,036,633, 5,065,552, and 5,147,018.
In variable damping devices of the above-described hydraulic types, however, it is difficult to obtain desired damping properties due to the oil temperature, the oil passage resistance, and various other factors. Seismic vibrations, as an object of the variable damping device, are unpredictable, resulting in complicated responses by the structure. As a consequence, small errors in control or design will greatly influence the damper control response to seismic vibrations. Therefore, there is need for a seismic control damping device which is not detrimentally influenced by oil temperature, oil leaks, oil line resistance, and the like.